webrtc: close without an await point; do not report an unknown path as direct

- close_webrtc is no longer async (hbb_common 88f965f), so the ten call
  sites in port_forward and io_loop - all inside select! arms or futures
  the UI can abandon - can no longer be cancelled mid-teardown, which
  left the pc unclosable and its session entry stranded. Client's own
  spawn_close_webrtc went with it: the runtime-teardown guard it existed
  for now lives in close_detached, so both Drop paths share one
  implementation.

- webrtc_relayed() returns None when no candidate pair is selected or
  the pc closed under a concurrent teardown, and both call sites read
  that as "not relayed", i.e. direct. A TURN-relayed session could
  therefore be shown to the user as peer-to-peer. Claiming a direct path
  needs evidence of one, so an unknown answer now counts as relayed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ExUfAkYbq8UC9pQCiLy8TQ

src/client/io_loop.rs

@@ -198,7 +198,7 @@ impl<T: InvokeUiSession> Remote<T> {
                     self.handle_disconnected(round);
                     // Close the WebRTC pc on this decline path too (no-op for TCP/WS); otherwise its
                     // pc lingers in the global session cache until ICE decays on its own.
-                    peer.close_webrtc().await;
+                    peer.close_webrtc();
                     return;
                 }
                 self.handler.update_direct(Some(direct));
@@ -348,7 +348,7 @@ impl<T: InvokeUiSession> Remote<T> {
                 // Close the WebRTC peer connection (if this session used it) so its pc is not left
                 // lingering in the global session cache after the session ends; dropping `peer`
                 // alone does not release it. No-op for TCP/WebSocket transports.
-                peer.close_webrtc().await;
+                peer.close_webrtc();
                 // Stop client audio server.
                 if let Some(s) = self.stop_voice_call_sender.take() {
                     s.send(()).ok();